Solving Instron Lab Results on Aluminium, PVC & Alumina Ceramic

[schmiggy]

Homework Statement

Hi,
I'm having a little trouble understanding some of the results retrieved from my engineering mechanic's classes recent compression experiment with an Instron machine. Specifically when plotting the data retrieved from the machine we see a non linear region before the linear elastic region and have no idea what it represents only that we are told to ignore it when calculating the modulus of elasticity and resilience.

So the questions are..
a) What could have caused the initial non linear shallow gradient before the linearly elastic region?
b) Why didn't the samples expand uniformly instead of bulking?

Also, the samples we used for the experiment were:
aluminium alloy (6060 T5)
PVC polymer
alumina ceramic (Al2O3)

The Attempt at a Solution

a) I'm honestly struggling to think of possible reasons for this.. could it have to do with the machine calibrating itself? For example, when setting up the machine, we set it such that the plate was very close to touching and then used the more sensitive control to position it such that it is only just touching. Or is it that the material deforms from lower stress initially until it reaches the linear elastic region? (if that at all made sense)
b) I'm lost for this one too, I can't even find reference to it in my materials textbook or online - to describe it by looking at it I guess I would call it 'reverse necking' or 'bulging', but I don't know what it's done that. My understanding is that with an evenly distributed load an object should deform uniformly - with that I would assume that the stress hasn't been distributed evenly.
Is it potentially from an uneven surface resultant from the material being cut? Material defects? Really need help with these two.. thanks!

If my answers appear silly or stupid, I apologise.. it only demonstrates my lack of understanding and need of help!

 

[KataKoniK]

Hello,

It's great to see that you are actively seeking to understand your results from the compression experiment. it is important to always question and understand the data we collect.

To answer your first question, the non-linear region before the linear elastic region could be caused by a few different factors. One possibility is that the material is undergoing an initial plastic deformation before reaching the linear elastic region. This could be due to imperfections in the material, such as microcracks or defects, which can cause stress concentrations and lead to non-linear behavior. Another possibility is that the machine itself may have some initial friction or stiffness that needs to be overcome before the material can start to deform linearly. This can be seen in the initial shallow gradient before the curve becomes linear.

For your second question, the bulking or reverse necking behavior could be due to a number of reasons. It could be a result of the material properties, such as its ductility or the presence of defects. It could also be affected by the loading conditions and how the load is applied to the material. For example, if the load is not evenly distributed, it could lead to non-uniform deformation and bulking. Another factor could be the surface of the material, as you mentioned. Any unevenness or roughness on the surface can affect the deformation behavior of the material.

I hope this helps to provide some insight into your results. It's important to continue to think critically about the data and consider all possible factors that may be influencing it. Keep up the good work!